Effect of Waveform of Controlled Strain in the Diametral Strain Controlled Low Cycle Fatigue

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As the plastic deformation is a time dependent process, the results of low cycle fatigue, which is caused by cyclic plastic deformation, can be subjected to the influence of form of strain cycle or stress cycle. From this point of view, diametral strain controlled low cycle fatigue tests were carried out on hour-glass shaped specimens of a quenched and tempered 60 kg/mm2 high tensile steel, waveform of strain cycle being changed in four conditions ; a sinusoidal wave, a triangular wave, and two trapezoidal waves of which hold time at the maximum and the minimum strain is 1 and 2 seconds. Cycling rate was 10 cpm irrespective of waveform, and strain ratio was -1.As a result of the tests, significant difference in crack initiation life was observed among the four strain waveforms in the range of high strain amplitude, while low cycle fatigue crack growth life was not affected. The order of crack initiation life was as follows the triangular wave, the sinusoidal wave, the trapezoidal wave of 1 second hold time at maximum and minimum strains, and the trapezoidal wave of 2 second hold time. The order is the inverse of the order of hold time around the maximum and minimum strain.The true stress-logarithmic strain hysteresis loops were composed from continuous records of load and deformation, and the relation between hysteresis energy and low cycle fatigue life was discussed in terms of effective damage factor. It was concluded that not only the hysteresis energy but also shape of the hysteresis loop is important in evaluating the role of the hysteresis, for the shape is much influenced by the wave form but the hysteresis energy is not.From observation of the surfaces of specimens by a x 50 microscope and a x 3 magnifier, it was confirmed that cracks of 0. 02 mm long were found by the microscope at about 10% of failure life, while cracks were found by naked eye at about 30% of failure life.